Transport network standards, such as the Synchronous Optical Network (SONET) and the Synchronous Digital Hierarchy (SDH), are used in time division multiplexed (TDM) networks in which link capacity is evenly divided temporally for efficient bandwidth management. The lowest bandwidth, or most granular, “high-order” switching unit of a SONET frame is a Synchronous Transport Signal, level-1 (STS-1) frame. Each STS-1 frame comprises nine rows of 90-columns transmitted in 125 microseconds (μs). As such, the STS-1 frame rate is 51.84 million bits per second (Mbps). Multiple SONET STS-1 frames can be multiplexed together to form higher rate frames. Currently, the defined SONET frame rates are STS-1, STS-3, STS-12, STS-48, STS-192, and STS-768. Some SONET frame designations have an appended “c” that indicates payload concatenation. An STS-N frame and a STS-Nc frame have the same frame rate, where N is 1, 3, 12, 48, 192, or 768.
In SDH, a Synchronous Transport Module, level-0 (STM-0) frame has the same frame rate and row-column structure as the SONET STS-1 frame. Higher levels of SDH frames are known as STM-N frames, where N can be 1, 4, 16, 64, and 256, corresponding to the same frame rates and row-column structures as SONET STS-3c, STS-12c, STS-48c, STS-192c, and STS-768c, respectively, as illustrated in TABLE I. The implementation of an STS-1/STM-0 time-space switch, also known as a SONET/SDH column switch, thus can treat the SONET columns and the SDH columns similarly.
TABLE IFrameFormatRateSONET FrameSDH FrameRowsColumns(Kbps)STS-1STM-099051,840STS-3cSTM-19270155,520STS-12cSTM-491,080622,080STS-48cSTM-1694,3202,488,320STS-192cSTM-64917,2809,953,280STS-768cSTM-256969,12039,813,120
A time-space switch with a lower level of switching granularity than STS-1/STM-0, however, may have to implement logic that distinguishes a SONET frame from an SDH frame. Each SONET STS-1 frame carries a payload in the synchronous payload envelope (SPE), which in turns carries “low-order” switching units know as virtual tributaries (VTs). In SDH, the low-order switching units are known as tributary units (TUs). TABLE II summarizes the frame sizes and rates for the virtual tributaries as follows:
TABLE IISONETVirtualSDHFrameTributaryTributaryFormatRate(VT)Unit (TU)RowsColumns(Kbps)VT1.5TU-11931,728VT2TU-12942,304VT3—963,456VT6TU-29126,912
Existing tributary time-space switches use the following solutions: (1) a full-blown column switch and (ii) limited data formats. The full-blown column switch means that any SONET/SDH column can be switched to any other column. The full-blown column switch is costly to implement in silicon. The limited data format approach limits the arrangements allowed in a frame to accommodate the switching. Some conventional switches support the North American standard only (SONET) but not the European and Asian standard (SDH). Some conventional switches preformat incoming data to a supported format. Some conventional switches can process VT1.5 but not VT2 traffic, although both are part of the SONET standard.